A standard microwave oven has been modified to enable rocking movement of a frozen bag of blood components during microwave thaw. With such a modified oven the warmer thawed portions of blood components flow over the frozen portions so as to distribute heat and minimize boiling of the blood components. It has been found however, that even with the rocking movement boiling of the blood can take place in the minimal areas of the bag of blood components such as the edges and in the tubular portions thereof. Various shielding devices have been used as microwave blood warmers in standard mirowave ovens. These shielding devices have been tested and found incapable of providing complete protection for the bag of blood components. Further, these shielding devices were found to be more difficult to use.